Field of Endeavor
The present invention relates to the technology of gas turbines. It refers to a method for operating a gas turbine, and in particular to a method for reliable operation of a gas turbine in changing fuel gas compositions. It further relates to a gas turbine unit useful for carrying out the method.
Brief Description of the Related Art
Usually gas turbines are fuelled with natural gas. Natural gas is mainly composed of CH4 (methane). Furthermore, natural gas also contains so-called non-CH4 components that can be diluting or enrichment substances. Examples of diluting substances are N2 (nitrogen) and CO2 (carbon dioxide). Enrichment substances usually are higher saturated hydrocarbons (C2+) such as C2H6 (ethane), C3H8 (propane), butane, etc.
Currently it is customary to characterize the quality, i.e., the composition of natural gas, by using two indices. These are the calorific value on one hand and the Wobbe index on the other hand. Up until now it furthermore has been customary to operate gas turbines with natural gas of consistent quality. Today gas supply companies are not yet able to guarantee consistent quality for the natural gas they supply.
As part of the liberalization of the natural gas market, natural gas suppliers increasingly attempt to optimize natural gas demand and natural gas prices. The result is that natural gas is obtained from different sources, is mixed and supplied to the consumers. This in turn results in a high degree of variability of the natural gas with regard to quality and/or composition.
It is clear that these varying properties of the natural gas influence the combustion process in the gas turbine combustor and consequently significantly influence the gas turbine operation. A change in fuel gas drives the gas turbine outside of its optimal operation window. This has an impact on the emission and pulsation behaviour and reduces the operational reliability. Therefore, power plant operators must be prepared for varying natural gas quality in the future.
It has already been proposed in the past (see, for example, U.S. Pat. Nos. 7,216,486 and 7,516,608) to measure the C2+ alkane content of the fuel gas and change the distribution of the fuel mass flows between the various combustion chambers and burners in accordance with variations of the fuel gas composition. An adjustment of the gas turbine operating parameters based on the rate of change in fuel gas energy content is disclosed in U.S. Pat. No. 7,854,110.
On the other hand, it has been proposed (see, for example, U.S. Pat. No. 7,484,352, and U.S. Pat. App. Pub. Nos. 2006/0040225 and U.S. 2009/0037029) to change the distribution of the fuel mass flows between the various combustion chambers and burners in dependence of pulsations being excited in the combustion chamber.
However, there is still a need for a better adaptation of the gas turbine operation to fast changing fuel gas compositions to improve the operational stability and reliability of a gas turbine.